The present invention may be more easily understood with reference to a telecommunication system in which individual subscribers are connected to a central office by metallic conductors having a limited frequency response. However, it will be apparent to those skilled in the art that the invention may be utilized in numerous other communication situations. The attenuation of the metallic conductors increases with frequency. This increase limits the data rates using conventional digital transmission techniques to about 64 Kbps. While this level is sufficient for conventional voice traffic and some data transmission needs, it would be advantageous to increase the throughput. For example, video applications require data rates in excess of 1 Mbps.
One method for increasing the throughput of such a data channel is to divide the bandwidth of the channel into a number of adjacent frequency bands. Each band is used to send a portion of the digital data. Those bands having the higher signal to noise ratios are used to transmit more bits than channels having smaller signal to noise ratios.
Consider one such frequency band. It will be assumed that the attenuation of the conductor at this frequency is essentially constant over the frequency band in question and that the noise levels in the frequency band are constant over time. Data is to be transmitted on this frequency band as "symbols" having some predetermined number of states. The maximum number of states will be determined by the signal to noise ratio in the frequency band. For example, assume that the maximum signal that can be sent to the receiver on the channel is 8 volts and the noise level in the channel is 0.5 volts. Then symbols having 8 states can be sent down the channel and correctly decoded. Hence, this channel can be used to send 3 bits on each transmission cycle.
Channel attenuation reduces the signal to noise ratio. There is always some maximum signal power that can be applied at the input side of the channel. A signal entering the input side of the channel will be reduced by the attenuation factor when it is received at the output side of the channel. However, the noise level on the channel is essentially independent of the attenuation. Hence, channels having higher attenuation will have lower signal to noise ratios. As a result, fewer bits can be sent on the higher attenuation channel.
If the channel attenuation is known and the noise levels remain constant, data symbols having the maximum number of states can be selected. In general, there are two sources of noise. The first source is relatively constant in time and depends on the environment in which the conductors are situated. The second source of noise is cross-talk between adjacent channels and conductors in cable over which the signals are being sent. In general, this noise source will change rapidly in time and will depend on the information being sent in the adjacent channels.
In prior art digital multi-carrier systems, the division of the channel into sub-bands is accomplished by utilizing a finite Fourier transform (FFT). Assume the channel is to be broken into M sub-channels. Each sub-channel is allocated part of the data. Denote the data value to be sent in the i.sup.th sub-channel by S.sub.i. Then, the data is processed by taking the FFT of the vector whose components are the S.sub.i. The resultant block of M inverse Fourier transform values is then sent on the channel. At the receiving end of the channel, the received values are transformed using the inverse FFT to recover the S.sub.i.
While this approach significantly improves the rate of data transmission on the channel, it is far from optimum. First, the FFT method for breaking the channel into sub-channels provides filters that have significant side-lobes. As noted above, side-lobes increase the noise in the channel and thereby limit the amount of data that can be sent in a sub-band.
In addition, burst noise can affect a significant number of data bits in this type of transmission system. In principle, each of the FFT transform values in a block is used in computing each of the S.sub.i. Hence, if one of the values is destroyed by a noise burst, the entire set of M symbols can be lost.
Broadly, it is the object of the present invention to provide an improved multi-carrier data transmission system.
It is a further object of the present invention to provide a multi-carrier transmission system having filters with reduced side-lobes relative to those obtained with FFT based systems.
It is a still further object of the present invention to provide a multi-carrier transmission system which ameliorates the effects of burst noise.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.